It is well documented that present bipolar active variable gain amplifiers have undesirable noise factor and inter-modulation characteristics with gain setting. The noise factor may vary by greater than 1 dB/dB with gain variation and intermodulation intercept point (IP) degrading with back-off. This results in an unacceptable degradation in carrier-to-noise-plus-inter-modulation power ratio, or C/(N+IM), for the first few (up to 6) dBs of gain back-off.
It is desirable from a signal handling perspective in a radio receiver to apply automatic gain control (AGC) as soon as possible. Applying AGC is intended to protect the internal stages from strong signal inter-modulation. Thus, the gain control is addressed at or as near as possible at the front of the radio frequency (RF) chain.
Conversely, it is desirable from an additive noise perspective to delay front-end AGC-controlled gain back-off as long as possible to minimize degradation in noise factor as the input signal increases and, hence required gain back-off also increases. For this reason, a classical AGC stage is frequently disposed behind input low-noise amplifier (LNA) gain protection, so minimizing degradation in overall noise factor.
Assume that an AGC stage is deployed in the front of the receiver. Further, assume that the AGC stage has sufficient gain to substantially protect the input referred additive noise from the internal stage noise. Then, ideally, the AGC stage of the receiver should have a NF characteristic that is substantially less than 1 dB/dB for the first few dBs of back-off, thus, delivering an improving carrier-to-noise power ratio (C/N). If the NF changed at 1 dB/dB with AGC back-off, then the C/N will never improve or, as is the case with prior art AGC implementations, which changes at greater than 1 dB/dB, the C/N would actually degrade.
A typical prior art AGC stage 10, shown in FIG. 1, is a stacked Gilbert cell with current steering. The AGC stage 10 may, for example, be deployed in an integrated circuit. The signal, Vin, is input to a gm stage 12. The resulting output current, Iout, which consists of a standing DC current and a signal current, Vin*gm arising from Vin, is steered through an un-degenerated long-tailed pair 14, between the load 16 and Vcc. The portion of Iout steered to the load 16 generates an output voltage, Vout; the resultant gain, Vout/Vin, is variable depending on the magnitude of the control voltage, Vcont. The AGC stage 10 displays the aforementioned undesirable NF variation with gain back-off. The AGC stage 10 of FIG. 1 is one of many possible circuit implementations to achieve this effect.
The NF issue can be partially alleviated by deploying an LNA in front of the AGC stage. However, such an arrangement compromises the inter-modulation performance because higher signals are incident at the AGC input. This can be partially alleviated by applying more current to the AGC stage. Neither of these options are optimum from performance, power, or silicon implementation perspectives.
Thus, there is a need for an AGC design that overcomes the shortcomings of the prior art.